1. Field of the Invention
The present invention relates to a fluorescent layer formation process, which is suitable for the fabrication of a color plasma display panel (PDP).
2. Related Prior Arts
Surface discharge PDPs have been commercially available as color display devices. The surface discharge PDPs have barrier ribs partitioning a discharge space on a pixel-by-pixel basis to define cells (display elements). Formed between the barrier ribs are fluorescent layers, which cover not only portions of a substrate surface extending parallel to a display surface but also side walls of the barrier ribs extending generally perpendicular to the display surface. The formation of the fluorescent layers is typically achieved by a screen printing method, because three-color (R, G, B) fluorescent layers should regularly be arranged. More specifically, three fluorescent pastes are sequentially applied onto fluorescent layer formation surfaces on a color-by-color basis with the use of three screens each having an aperture pattern corresponding to color arrangement. The three fluorescent pastes thus applied are subjected to a predetermined drying process, and then simultaneously baked. A fluorescent substance to be used for the PDPs is a luminophor of ultraviolet ray excitation type. For example, (Y,Gd)BO.sub.3 :Eu.sup.3+ is used as an R-fluorescent substance, Zn.sub.2 SiO.sub.4 :Mn or BaAl.sub.12 O.sub.19 :Mn is used as a G-fluorescent substance, and 3(Ba,Mg)O.8Al.sub.2 O.sub.3 :Eu.sup.2+ is used as a B-fluorescent substance.
For the formation of the fluorescent layers covering the side walls of the barrier ribs, it is required to blend a resin with the respective fluorescent substance to obtain a fluorescent paste. The resin blended in an appropriate amount increases the viscosity of the paste to keep the paste adhering onto the side walls of the barrier ribs during the drying process. The resin also serves to increase the filling density of fluorescent substance particles.
In a conventional PDP fabrication process, the fluorescent pastes are each prepared in the following manner: (1) ethyl cellulose as a resin of a viscosity builder component is dissolved in a solvent for preparation of a vehicle having a high viscosity (about 400 poises) ; and (2) a fluorescent substance is dispersed in the vehicle.
The PDPs employing fluorescent substances for color display generally suffer reduction in the color reproducibility due to deterioration of the fluorescent substances. For prevention of the deterioration of the fluorescent substances, the baking temperature for the formation of the fluorescent layers should be reduced as much as possible.
To burn out ethyl cellulose contained in the fluorescent paste, the temperature for the baking of the pastes should be controlled at not lower than 450.degree. C. The deterioration of the fluorescent substance becomes more remarkable with an increase in the baking temperature. More specifically, the valence of europium (Eu) serving as an active substance changes, making it difficult for the fluorescent substance to emit light. Ethyl cellulose is a natural material which contains a relatively large amount of inorganic substances. The inorganic substances are not burnt out to remain as impurities in the fluorescent layers. Further, the compositions of the fluorescent layers are slightly different depending on raw materials to be used.
Known as a synthetic resin usable as the paste material is an acrylic resin which is susceptible for the pyrolysis to ethyl cellulose. An attempt has been made to form fluorescent layers by employing a high viscosity fluorescent paste prepared with the use of the acrylic resin in substantially the same manner as with the use of ethyl cellulose. However, when the fluorescent paste is printed on a substrate masked with a mesh having an emulsion mask (screen mask) and then the screen mask is removed from the substrate, the fluorescent paste strings between the substrate and the screen mask, and is locally brought back from the substrate to the screen mask so that the resulting paste layer does not have a uniform thickness. In the worst case, the stringing is so heavy that no fluorescent paste remains on the substrate. If the viscosity of the paste is reduced to avoid the stringing, the fluorescent substance precipitates in the paste, making it impossible to afford a fluorescent layer exhibiting a uniform luminance.